Process for construction of high temperature capacitor



Feb. :25, 1969 R. J. DIEFENDORF PROCESS FOR CONSTRUCTION OF HIGHTEMPERATURE CAPACITOR Original Filed 001;. 21, 1964 His Aflomey.

United States Patent 3,429,020 PROCESS FOR CONSTRUCTION OF HIGHTEMPERATURE CAPACITOR Russell J. Diefendorf, Ballston Spa, N.Y.,assignor to General Electric Company, a corporation of New York Originalapplication Oct. 21, 1964, Ser. No. 405,421, now Patent No. 3,335,345,dated Aug. 8, 1967. Divided and this application May 23, 1967, Ser. No.640,644 US. Cl. 2925.42 6 Claims Int. Cl. H0lg 13/00 ABSTRACT OF THEDISCLOSURE Preparation of a capacitor operable at temperatures up toabout 1000 C. is described. The specific method disclosed employspyrolytic decomposition of different gaseous media in sequence toproduce alternate layers of pyrolytic graphite and pyrolytic boronnitride.

This application is a division of US. patent application Ser. No.405,421, Diefendorf, filed Oct. 21, 1964 (now US. Patent 3,335,345) andassigned to the assignee of the instant application.

Present high temperature capacitors operate at temperatures up to about600 C. maximum. Capacitors, which operate at temperatures up to about1000 C. are desirable for employment in electronic circuitry operatingat such elevated temperatures. My invention is directed to such animproved capacitor which operates at elevated temperatures up to about1000 C.

It is an object of my invention to provide an improved capacitor whichis operable up to 1000 C.

It is another object of my invention to provide an improved capacitorwhich employs pyrolytic graphite electrodes.

It is a further object of my invention to provide an improved capacitorwhich employs a boron nitride dielec tric between the electrodes.

It is a still further object of my invention to provide an improvedcapacitor which is formed pyrolytically.

In carrying out my invention in one form, a capacitor comprises a firstpyrolytic graphite electrode, a pyrolytic boron nitride dielectricpositioned on the first electrode, a second pyrolytic graphite electrodepositioned on the dielectric, and an electrical lead connected to eachof the electrodes.

These and various other objects, features, and advantages of theinvention will be better understood from the following description takenin connection with the accompanying drawing in which:

FIGURE 1 is a sectional view of an improved capacitor embodying myinvention;

FIGURE 2 is a sectional view of a modified capacitor embodying myinvention; and

FIGURE 3 is a sectional view of apparatus for forrning the electrode anddielectric of the capacitor.

Pyrolytic graphite is defined as a material made from carbonaceousgasesfby thermal decomposition or from a carbonaceous material byevaporation and deposition on a surface. Pyrolytic boron nitride isdefined as a material made from gases by thermal decomposition or frommaterials formed by evaporation and deposition on a surface. In bothpyrolytic graphite and pyrolytic boron nitride, planar crystallites havea preferred orientation and are arranged so that their layers aregenerally parallel to the deposition surface. Pyrolytic graphite orpyrolytically-deposited graphite is an electrically conductive materialwhile pyrolytic boron nitride or pyrolyticallydeposited boron nitride isan electrically insulating material which is suitable for a dielectric.

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In FIGURE 1 of the drawing, a capacitor is shown generally at 10 whichcomprises a pair of pyrolytic graphite electrodes 11 and a pyrolyticboron nitride dielectric 12 positioned between and in contact with theopopsite surfaces of electrodes 11. An electrical lead 13 is shownconnected by any suitable manner to each of the electrodes 11.

Each pyrolytic graphite electrode 11 and pyrolytic boron nitridedielectric 12 is shown in the form of a layer, each of the layers beingparmallel to the other layers in capacitor 10 and each layer composed ofa plu rality of individual layers of its respective material. Dielectriclayer 12 is positioned or stacked on electrode 11 while second electrode11 is positioned on layer 12 to form the capacitor structure. Layer 12of pyrolytic boron nitride extends beyond the respective edges ofpyrolytic graphite electrodes 11 to prevent arcing over between theelectrodes. Electrodes 11 are cut to smaller dimensions than thedielectric 12 so that dielectric 12 extends beyond the respective edgesof electrodes 11. Additionally, if it is desired electrodes 11 anddielectric 12 are initially of the same dimensions. The edges ofelectrodes 11 are then etched away by any suitable means such asoxidizing. Since pyrolytic boron nitride oxidizes at a much slower ratethan pyrolytic graphite, this method is successful to produce thegreater diameter for dielectric 12. When this structure is stacked,electrode 11 and dielectric 12 are then held together in any suitablemanner such as by clamping together or by bonding electrodes 11 to thedielectric 12. Leads 13 are then affixed to electrodes 11 to formcapacitor 10.

Additionally, FIGURE 1 is also viewed as a capacitor 10 comprising afirst pyrolytic graphite electrode 11 on the upper surface of which ispyrolytically deposited a boron nitride dielectric 12. A pyrolyticgraphite electrode 11 is then deposited on the upper surface of boronnitride dielectric 12. Leads 13 are then attached to respectiveelectrodes 11. If the boron nitride dielectric and the pyrolyticgraphite electrodes are pyrolytically deposited to form these layers,the edges of the electrodes are then oxidized so that the boron nitridedielectric extends over the edges of the electrode to prevent arcing.

In FIGURE 2 of the drawing there is shown a modified capacitor orcapacitor roll 14 which comprises a first layer of pyrolytic graphite11, a second layer of pyrolytic boron nitride 12 on the surface of layer11, a second layer of pyrolytic graphite 11 on boron nitride layer 12,and a layer of electrical insulation in the form of a pyrolytic boronnitride 12 on the second pyrolytic graphite layer, and an electricallead 13 connected to each of the pyrolytic graphite layers.

As in FIGURE 1 of the drawing, the respective pyrolytic graphite layers11 are the electrodes of the structure while first boron nitride layer12 is the dielectric. The second layer of pyrolytic boron nitride 12 isa layer of electrical insulation. These layers are stacked in a similarmanner to the device shown in FIGURE 1. The layers are held together ina similar fashion and rolled counterclockwise from left to right inFIGURE 2 whereby insulation 12 is provided between each layer assemblyof electrodes 11 and dielectric 12. Additionally, each of these layersis pyrolytically deposited to form capacitor roll 14 in a manner similarto that described above for FIGURE 1. The electrodes are reduced indimensions so that the dielectric and the insulation extend over theedges of the electrodes. Electrical leads are aflixed then to electrodes11.

While it is not shown in FIGURE 2 of the drawing, a plurality ofcapacitor rolls 14 as shown in FIGURE 2 are rolled into tubular form andpositioned within a container. Electrical insulation is positioned thenbetween the exterior surfaces of the rolls and the interior surface ofthe container. The pyrolytic graphite layers of each of the rolls areconnected electrically, and a pair of terminal leads are connected toassociated pyrolytic graphite lay ers and extend through the containerto form a capacitor.

I discovered that a capacitor which would operate at temperatures up toabout 1000 C. could be formed from a first pyrolytic graphite electrode,a pyrolytic boron nitride dielectric positioned on the pyrolyticgraphite electrode, a second pyrolytic graphite electrode positioned onthe dielectric, and an electrical lead connected to each of theelectrodes.

I found further that such a capacitor could comprise a first pyrolyticgraphite electrode, a pyrolytically-deposited boron nitride dielectricon one surface of the electrode, a pyrolytically-deposited graphiteelectrode on the boron nitride dielectric, and an electrical leadconnected to each of the electrodes. Thus, such a capacitor constructionwould include the stacking of at least two pyrolytic graphiteelectrodes, with a pyrolytic boron nitride dielectric therebetween orsuch a structure wherein at least two pyrolytically-deposited graphiteelectrodes had an alternate pyrolytically-deposited boron nitridedielectric therebetween.

Additionally, I found that a capacitor roll could comprise a layer ofpyrolytic graphite, a layer of pyrolytic boron nitride on the pyrolyticgraphite layer, a second layer of pyrolytic graphite on the boronnitride layer, a layer of electrical insulation on the second pyrolyticgraphite layer, and an electrical lead connected to each of thepyrolytic graphite layers. Such a structure is then rolled and stackedwith a number of similar rolls in an insulated container to provide acapacitor.

Additionally, such a roll could be formed by the pyrolytic deposition ofthe graphite and the boron nitride. The layer of insulation of such astructure is preferably pyrolytic boron nitride.

Each of the above capacitor structures has a high dielectric strengthand a high breakdown. voltage. Additionally, these capacitors have theadvantage which is lacking in a silver-mica capacitor in that the lattertype of capacitor is useful up to a maximum temperature of about 580 C.My capacitor will operate at lower temperature ranges and also is highlydesirable in that it will operate up to temperatures of about 1000 C.

In FIGURE 3 of the drawing, apparatus is shown generally at whichcomprises a central heating element 16 of high temperature material inthe form of a hollow tubular configuration of commercial graphite. Othersolid or hollow configurations of high temperature material may also beemployed for the heating element. At each end of heating element or tube16 is positioned a pyrolytic graphite electrode 17 in mechanical andelectrical contact therewith. Each of these electrodes 17 is shown inthe form of a ring with a flange fitting against the end of tube 16.However, any suitable electrode configuration can be employed. Agraphite electrode 18 which has a larger length-to-area ratio for theelectric current path is positioned adjacent electrode 17 and inelectrical contact therewith. In this figure of the drawing, pyrolyticgraphite electrode 17 and graphite electrode 18 form an electrodeassembly for constant heating of graphite tube 16 during the operationof furnace apparatus 15.

Tube 16 is insulated from heat loss by suitable insulation 19 in theform of a blanket of graphite felt or thermal black. Since furnace 15 isadapted to be employed as a vacuum furnace, an outer metallic casing 20,for example, of brass, is positioned around insulation 19. Casing 20 issuitably cooled by water coils 21 surrounding the casing. At oppositeends of the casing 20, there is provided end cover structures 22 and 23which each include a water-cooled electrode 24 in electrical contactwith electrode 18. Electrode 24 is threaded 'at its inner periphery 25.An inner ring member 26 is threaded to threads of electrode 24. Aplurality of bolts 29 are inserted through the openings in cover 22 andthreaded into openings in upper flanges 28 of casing 20 to mechanicallyposition upper graphite electrode 18 against electrode 17. Upperelectrode 18 is also provided with a plurality of openings 29 to providepassageways to insulation 19. A plate 30 is affixed as by screws 31 tothe upper surface of member 25. An 0 ring 32 is provided near the outerperiphery of plate 30 to produce an effective seal. A viewing window 33is shown positioned centrally in plate 30.

An electrical lead 34 is shown in electrical contact with water-cooledelectrode 24. The lead is connected to one terminal of a power source(not shown). Cover 22 is secured to the upper end of casing 20 by meansof a plurality of bolts 27 which are inserted through a plurality ofopenings in cover 22 and which are threaded in threaded openings in aflange 28 on the upper end of casing 20. Electrically and thermallyinsulating material 35 is provided between cover 22 and flange 28, andwithin and surro nding openings in cover 22.

Lower cover 23 has a water-cooled electrode 24 near its outer periphery.Cover 23 has a central plate portion 36 with an aperture 37 thereincentrally located into which a feed line 38 is afiixed. A plurality ofbolts 27 are inserted through openings in lower cover 23 and arethreaded in threaded openings in lower flange 28 of casing 22 to securelower cover 23 to casing 22. Insulaltion 35 can also be provided betweencover 23 and flange 28 and within and surrounding the openings in cover23. A second lead 39 is connected to water-cooled electrode 24 in cover23 and to the other terminal of the power source (not shown) to completethe electrical circuit to the furnace. The inner periphery of thewater-cooled electrode 24 is tapered inwardly towards casing 20 toprovide support for graphite electrode 18 which is tapered in similarfashion and fits thereagainst.

Feed line 38 is connected to material sources (not shown), for example,for boron containing materials, nitrogen containing materials, andhydrocarbon materials which it is desired to introduce into a graphitetube 40 positioned within tube 16. With such a tube 40 in position, thegas flows from inlet line 38 into the interior of tube 40. At the upperend of furnace 15 an opening 41 is provided, for example, through plate30 of cover 22. A tube 42 is positioned in opening 41 and connected to apump 43 to provide for evacuation of furnace 15 to a desiredsubatmospheric condition during operation. Additional feed lines (notshown) may be employed and connected to separate material sources.

In the operation of furnace 15 shown in FIGURE 3 of the drawing, a thintube 16 of graphite infiltrated with pyrolytic graphite is employed. Apair of pyrolytic graphite electrodes are applied at opposite ends oftube 16. Commercial graphite electrodes 18 are positioned in contactwith electrodes 17 to formelectrode assemblies. A blanket of insulationsuch as carbon felt 19 is wrapped around the tube 16. This structure isfitted into casing 20, for example, through an open upper end of thecasing with lower cover 23 already attached. Subsequently, bolts 27 areinserted through the openings in cover 22 and threaded into the openingsin flange 28 of casing 20. The chamber atmosphere is reduced preferablyto the lowest obtainable vacuum prior to admitting a gas, although thedeposition process can be carried out over a wide range of chamberpressures such as 0.1 millimeters of mercury to 760 millimeters ofmercury, at various gas flow rates. If desired, the interior of tube 40may be purified by a preheat treatment at a temperature of at least 2350C. This preheating is described in US. Patent 3,138,435 issued June23,1964, and assigned to the same assignee as the present application.

In one method of forming a capacitor in accordance with the presentinvention, pyrolytic graphite electrodes are formed by depositing alayer of pyrolytic graphite on the interior surface of tube 40, removingsubsequently this layer from tube 40, for example, by chemically etchingaway the tube from the pyrolytic graphite layer, and

machining the layer to appropriate sizes to provide pyrolytic graphiteelectrodes.

In the formation of this pyrolytic graphite layer on the interiorsurface of tube 40, suificient power is generated to heat tube 16 andtube 40 rapidly to a temperature in the range of 1200" C. to 2100 C.This temperature range is desirable to produce a uniform pyrolyticgraphite layer which is removed readily from the plates. A carbonaceousgas, such as methane, is fed through appropriate metering devices (notshown) and a preheater (not shown) and feed line 38 into the interior oftube 40. While a carbonaceous gas, such as methane, ethane, propane,acetylene, benzene, carbon tetrachloride, or cyanogen is employed, thecarbonaceous material can also be in liquid or solid form which is fedfrom the source to a preheater from conversion to a carbon vapor. Thegas is decomposed to a carbon vapor which deposits as a pyrolyticgraphite layer on the interior surface of tube 40. The deposited layeris composed of a plurality of individual generally parallel pyrolyticgraphite layers.

Additionallly, the gas can be preheated from a separate heat source tothe desired temperature to provide a carbon vapor which flows throughfeed line 38 into tube 40. During the operation of apparatus 15,temperatures are recorded by an optical pyrometer (not shown) which isviewed through window 33 in cover 22 of apparatus 15. At the abovementioned temperatures, it is possible to form 20 to 40 mils ofpyrolytic graphite an hour. After the desired thickness of the pyrolyticgraphite layer is obtained, the gas flow is stopped, the pressure isdecreased further, and the apparatus is allowed to cool to roomtemperature. The pressure is increased subsequently to atmosphericpressure, and cover 22 is unbolted from casing 15. The apparatus isdisassembled to remove coated tube 40 from tube 16 in furnace 15. Thepyrolyticallydeposited graphite layer is then removed from the interiorsurface of tube 40 as discussed above. The pyrolyticallydeposited layeris then machined to the desired size to provide a plurality of graphiteelectrodes 11.

The same apparatus shown in FIGURE 3 is then employed to provide apyrolytically-deposited boron nitride layer on the interior surface oftube 40 in a similar manner. Power is generated to heat tube 16 and tube40 rapidly to a temperature in the range of 1400 C. to 2000 C. prior toadmitting the gas. A boron and nitrogen component gas, such asB-trichlorborazole, which has been heated to a temperature of 80 C., isfed through suitable metering devices (not shown) and feed line 38 intothe interior of tube 40. The gas is decomposed to a vapor which depositsas a pyrolytic boron nitride layer on the interior surface of tube 40.This layer is composed of a plurality of individual generally parallelboron nitride layers. This temperature range and the above pres surerange are desirable to produce individual fine-grain pyrolytic boronnitride layers. In addition to B-trichlorborazole, B N H Cl whichprovides both the boron and nitrogen components, various startingmaterials such as BCl or B H are suitable to provide the boron compoundwhile NH is suitable to provide the nitrogen compound. Additionally, thegas can be preheated from a separate heat source to the desiredtemperature to provide a vapor which flows through feed line 38 intotube 40.

After the desired thickness of the pyrolytic boron nitride layer isobtained, the gas flow is stopped, the pressure is decreased further,and the apparatus is allowed to cool to room temperature. The pressureis increased subsequently to atmospheric pressure, and cover 22 isunbolted from casing 15. The apparatus is disassembled to remove coatedtube 40 from tube 16 in furnace 15. The deposited layer is then removedfrom the interior surface of tube 40 as discussed above. The pyrolyticboron nitride layer is then machined to appropriate sizes to provide aplurality of dielectrics 12.

After both the pyrolytic graphite electrodes have been machined to size,and the boron nitride layer has been machined to size, the capacitor inFIGURE 1 is assembled. A pair of pyrolytic graphite electrodes 11 have apyrolytic boron nitride dielectric 12 positioned therebetween and incontact with the surfaces of electrodes 11. An electrical lead 13 isconnected to each of the electrodes 11 in any suitable manner. As isshown in FIG- URE l of the drawing, the pyrolytic boron nitridedielectric 12 extends beyond the respective edges of electrodes 11 toprevent arcing between the electrodes. This is accomplished as discussedabove by cutting the dielectric to a larger size than the electrodes.The electrodes 11 and dielectric 12 are clamped together or bondedtogether in any suitable manner. If electrodes 11 and dielectric 12 areinitially of the same dimensions, the electrodes are etched away attheir edges as by oxidizing to provide the dielectric layer extendingbeyond the edges of the electrodes.

In addition to stacking the electrodes and dielectric to form acapacitor shown in FIGURE 1, the apparatus in FIGURE 3 may be employedalso to produce directly this type of structure wherein the layers arepyrolytically deposited to form a capacitor. As it was discussed abovein connection with the operation of the apparatus shown in FIGURE 3, acapacitor construction such as shown in FIGURE 1 may be produced in acontinuous manner by first depositing pyrolytic graphite from acarbonaceous vapor onto the interior surface of tube 40 to a desiredthickness. The gas is then stopped. Since the deposition temperaturesfor pyrolytic graphite and pyrolytic boron nitride overlapsubstantially, the temperature is maintained constant for the entireprocess or lowered to a temperature in the range of 1400 C. to 2000 C. Aboron and nitrogen component gas is fed through feed line 38 to theinterior of tube 40 to produce a pyrolytically-deposited boron nitridelayer on the first pyrolytic graphite layer. The gas is then stopped.Similarly, the temperature is maintained constant or raised in thetemperature range of 1200 C. to 2100" C. The initial carbonaceous gas isthen flowed into tube 40. A second pyrolytic graphite layer is thendeposited on the pyrolyticallydeposited boron nitride layer.

As it was described above after the deposition is completed, tube 40 isremoved from the apparatus and a composite structure consisting of afirst layer of pyrolytic graphite, a second layer ofpyrolytically-deposited boron nitride on the graphite layer, and asecond layer of pyrolytic graphite deposited on the boron nitride layerare removed from the interior surface of tube 40, for example, bychemically etching away tube 40 in a concentrated solution of sulfuricacid. The composite structure is then machined to appropriate sizes toprovide a plurality of capacitor structures. The opposite pyrolyticgraphite layers, which are the electrodes for each capacitor, are etchedaway so that the boron nitride layer extends beyond the respective edgesof the electrodes to prevent arcing. A pair of leads are then connectedto the electrodes to complete the capacitor structure.

As it was described above in connection with FIG- URE 2 of the drawing,a capacitor or capacitor roll is formed from a stack comprising a layerof pyrolytic graphite, a layer of boron nitride positioned on thepyrolytic graphite layer, a second layer of pyrolytic graphitepositioned on the boron nitride layer, a layer of electrical insulation,such as boon nitride positioned on the second pyrolytic graphite layer,and an electrical lead connected to each of the pyrolytic graphitelayers. This material is produced in the apparatus in FIGURE 3 and thelayers stacked upon one another and clamped or bonded in any suitablemanner. The edges of the dielectric and the insulation extend over theedges of the electrodes to prevent arcing and are provided in thismanner by etching away of the electrodes or different dimensioning forthe electrodes, the dielectric and the insulation.

The capacitor or capacitor roll 14 which is shown in FIGURE 2 is thenrolled counterclockwise from left to right as shown in the figurewhereby installation 12 is provided between each layer assembly ofelectrodes 11 and dielectric 12. A plurality of these rolls may bepositioned in an insulated container wherein the rolls are connectedelectrically, and a pair of termina leads are connected to the pyrolyticgraphite electrodes and extends through the container forming acapacitor.

In addition to stacking the electrodes, dielectric, and insulation isthe form of a capacitor shown in FIGURE 2, apparatus in FIGURE 3 mayalso be employed to produce directly a capacitor structure wherein thelayers are pyrolytically-deposited to form a capacitor or capacitor roll14. As it was discussed above in connection with the operation of theapparatus shown in FIGURE 3, the capacitor construction as shown inFIGURE 2 is produced in a continuous manner by first depositingpyrolytic graphite from a carbonaceous vapor onto the interior surfaceof tube 40 to a desired thickness. The gas is then stopped. Thetemperature is maintained constant or lowered to a temperature in therange of 1400 C. to 2000 C. A boron and nitrogen component gas is fedthrough feed line 38 to the interior of tube 40 to produce apyrolytically-deposited boron nitride layer on the first pyrolyticgraphite layer. The gas is then stopped. The temperature is maintainedconstant or raised to a temperature in the range of 1200 C. to 2100 C.The initial carbonaceous gas is then flowed into tube 40. A secondpyrolytic graphite layer of desired thickness is then deposited on thepyrolytically-deposited boron nitride layer. The gas is then stopped.The temperature is maintained constant or lowered to a temperature inthe range of 1400 C. to 2000 C. The initial boron and nitrogen componentgas is fed through feed line 38 to tube 40 to produce apyrolytically-deposited boron nitride layer of desired thickness on thesecond pyrolytic graphite layer.

As it was described above after the deposition is completed, tube 40 isremoved from the apparatus and a composite structure consisting of afirst layer of pyrolytic graphite, a second layer ofpyrolytically-deposited boron nitride on the graphite layer, a secondlayer of pyrolytic graphite deposited on the boron nitride layer, and asecond layer of pyrolytic boron nitride deposited on the secondpyrolytic graphite layer are removed from the interior surface of tube40, for example, by chemically etching away tube 40 in a concentratedsolution of sulfuric acid. The composite structure is machined toappropriate sizes to provide a plurality of capacitor structures. Thepair of pyrolytic graphite layers, which are the electrodes for thecapacitor, are etched away so that the boron nitride layers extendbeyond the respective edges of the electrodes to prevent arcing. A pairof leads are then connected to the electrodes to complete the capacitorstructure. This capacitor 14 or capacitor roll is then rolledcounterclockwise from left to right as shown in FIGURE 2, wherebyinsulation 12 is provided between each layer assembly of electrodes 11and dielectric 12. If desired, a plurality of these rolls are positionedin an insulated container and connected electrically. A terminal lead isconnected to each of the pyrolytic graphite electrodes and extendsthrough the container to form a capacitor.

Examples of capacitors formed in accordance with the present inventionwere as follows:

Example I Apparatus was set up in accordance with FIGURE 3 of thedrawing wherein the deposition tube was composed of commercial graphite.The cover was bolted to the casing and the chamber defined by thedeposition tube was reduced to a pressure of 0.010 millimeter of mercuryby the pump. Power was supplied which heated the tube and tube passageto an uncorrected optimal pyrometer temperature reading of about 1650 C.A carbonaceous gas in the form of methane was supplied at a rate of 0.5cubic foot per hour at a pressure of 1000 millimeters of mercury throughthe feed line to the interior of the tube. The gas was formed into acarbon vapor in the tube which vapor was deposited uniformly aspyrolytic graphite on the interior surface of the tube as it flowedthrough the tube at a pressure of approximately 0.5 millimeter ofmercury. Under the above conditions, a pyrolytic graphite layer of 0.6mil thickness was formed on the interior surface of the tube after about11 minutes. This layer was the first pyrolytic graphite electrode of thesubsequent capacitor.

The flow of methane gas was then stopped, the temperature was maintainedat an uncorrected optical pyrometer temperature reading of about 1650C., and a boron and nitrogen component gas in the form of borontrichloride in gaseous form was supplied at a rate of 0.05 cubic footper hour to the feed line to the interior of the tube. A second feedline supplied ammonia at a rate of 0.15 cubic foot per hour to theinterior of the tube. These gases were mixed in the interior of theheated tube. The gases were for-med into a vapor in the tube which vaporwas deposited uniformly as boron nitride on the interior surface of thetube as it flowed through the tube at a pressure of approximately 0.175millimeter of mercury. Under these conditions, a pyrolytic boron nitridebody of 0.5 mil thickness was formed on the surface of each of thepyrolytic graphite layers. The deposition time was 15 minutes.

The flow of boron trichloride and ammonia was then stopped, thetemperature was maintained at 1650 C. and the other initial conditionsfor the deposition of pyrolytic graphite were then employed. Methane wasflowed again under its initial conditions through the feed line to thetube to deposit a second layer of pyrolytic graphite on the surface ofthe pyrolytic boron nitride layer. Under these conditions, a secondpyrolytic graphite layer of 0.7 mil thicknes was formed on the surfaceof the pyrolytic boron nitride layer after about 15 minutes.

The gas flow was then stopped, the pressure was decreased further andthe assembly within the casing was allowed to cool to room temperature.The pressure was increased subsequently to atmospheric pressure and thecover was removed from the casing to provide access to the tube. Thecoated tube was then removed from the casing. The layered structure ofpyrolytically-deposited material was then removed from the interiorsurface of the tube by etching away chemically the tube in aconcentrated solution of sulfuric acid. Each of these layered structurescomprises a first layer of pyrolytic graphite, a layer of pyrolyticboron nitride thereon, and a second layer of pyrolytic graphite thereonand was then machined to the desired size to provide a plurality ofcapacitors.

The edges of the pyrolytic graphite layers were etched away by theoxidizing whereby the boron nitride layer extends over the edges of thepyrolytic garphite layers to prevent arcing therebetween. A pair of leadwere then afiixed to the pyrolytic graphite layers or electrodes. Thus,a pyrolytically deposited capacitor structure was provided by thismethod.

Example II Apparatus was set up in accordance with FIGURE 3 of thedrawing wherein the deposition tube was composed of commercial graphite.The cover was bolted to the casing and the chamber defined by thedeposition tube was reduced to a pressure of 0.010 millimeter of mercuryby the pump. Power was supplied which heated the tube and tube passageto an uncorrected optimal pyrometer temperature reading of about 1925 C.A carbonaceous gas in the form of methane was supplied at a rate of 0.5cubic foot per hour at a pressure of 1000 millimeters of mercury throughthe feed line to the interior of the tube. The gas was formed into acarbon vapor in the tube which vapor was deposited uniformly aspyrolytic graphite on the interior surface of the tube as it flowedthrough the tube at a pressure of approximately 0.5 millimeter ofmercury. Under the above conditions, a pyrolytic graphite layer of 6 milthickness was formed on the interior surface of the tube after about 124minutes. This layer was the first pyrolytic graphite electrode of thesubsequent capacitor.

The fiow of methane gas was then stopped, the temperature, which haddropped during the initial deposition, was at an uncorrected opticalpyrometer temperature reading of about 1660 C., and a boron and nitrogencomponent gas in the form of boron trichloride in gaseous form wassupplied at a rate of 0.1 cubic foot per hour to the feed line to theinterior of the tube. A second feed line supplied ammonia at a rate of0.3 cubic foot per hour to the interior of the tube. These gases weremixed in the interior of the heated tube. The gases were formed into avapor in the tube which vapor was deposited uniformly as boron nitrideon the interior Surface of the tube as it flowed through the tube at apressure of approximately 0.3 millimeter of mercury. Under theseconditions, a pyrolytic boron nitride body of 1.0 mil thickness wasformed on the surface of the pyrolytic graphite layer. The depositiontime was 10 minutes.

The flow of boron trichloride and ammonia was then stopped, thetemperature was maintained at 1660 C., and the other initial conditionsfor the deposition of pyrolytic graphite were then employed. Methane wasfiowed again under its initial conditions through the feed line to thetube to deposit a second layer of pyrolytic graphite on the surface ofthe pyrolytic boron nitride layer. Under these conditions, a secondpyrolytic graphite layer of 9 mil thickness was formed on the surface ofthe pyrolytic boron nitride layer after about 240 minutes.

The gas fiow was then stopped, the pressure was decreased further andthe assembly within the casing was allowed to cool to room temperature.The pressure was increased subsequently to atmospheric pressure and thecover was removed from the casing to provide access to the tube. Thecoated tube was then removed from the casing. The layered structure ofpyrolytically-deposited material was then removed from the interiorsurface of the tube by etching away chemically the tube in aconcentrated solution of sulfuric acid. Each of these layered structurescomprised a first layer of pyrolytic graphite, a layer of pyrolyticboron nitride thereon, and a second layer of pyrolytic graphite thereonand was then machined to the desired size to provide a plurality ofcapacitors.

The edges of the pyrolytic graphite layers were etched away by theoxidizing whereby the boron nitride layer extends over the edges of thepyrolytic graphite layers to prevent arcing therebetween. A pair ofleads were then afiixed to the pyrolytic graphite layers or electrodes.Thus, a pyrolytically deposited capacitor structure was provided by thismethod.

Example 111 Apparatus was set up in accordance with FIGURE 3 of thedrawing wherein the deposition tube was composed of commercial graphite.The cover was bolted to the casing and the chamber defined by thedeposition tube was reduced to a pressure of 0.010 millimeter of mercuryby the pump. Power was supplied which heated the tube and tube passageto an uncorrected optimal pyrometer temperature reading of about 1450 C.A carbonaceous gas in the form of methane was supplied at a rate of 0.5cubic foot per hour at a pressure of 1000 millimeters of mercury throughthe feed line to the interior of the tube. The gas was formed into acarbon vapor in the tube which vapor was deposited uniformly aspyrolytic graphite on the interior surface of the tube as it fiowedthrough the tube at a pressure of approximately 0.5 millimeter ofmercury. Under the above conditions, a pyrolytic graphite layer of 1.2mil thickness was formed on the interior surface of the tube after about190 minutes. This layer was the first pyrolytic graphite electrode ofthe subsequent capacitor.

The fiow of methane gas was then stopped, the temperature was maintainedat an uncorrected optical pyrometer temperature reading of about 1450C., and a boron and nitrogen component gas in the form of borontrichloride in gaseous form was supplied at a rate of 0.1 cubic foot perhour to the feed line to the interior of the tube. A second feed linesupplied ammonia at a rate of 0 .3 cubic foot per hour to the interiorof the tube. These gases were mixed in the interior of the heated tube.The gases were formed into a vapor in the tube which vapor was depositeduniformly as boron nitride on the interior surface of the tube as itflowed through the tube at a pressure of approximately 0.115 millimeterof mercury. Under these conditions, a pyrolytic boron nitride body of0.3 mil thickness was formed on the surface of the pyrolytic graphitelayer. The deposition time was 5 minutes.

The fiow of boron trichloride and ammonia was then stopped, thetemperature was maintained at 1450 C., and the other initial conditionsfor the deposition of pyrolytic graphite were then employed. Methane wasflowed again under its initial conditions through the feed line to thetube to deposit a second layer of pyrolytic graphite on the surface ofthe pyrolytic boron nitride layer. Under these conditions, a secondpyrolytic graphite layer of 0.9 mil thickness was formed on the surfaceof the pyrolytic boron nitride layer after about 132 minutes.

The gas flow was then stopped, the pressure was decreased further andthe assembly within the casing was allowed to cool to room temperature.The pressure was increased subsequently to atmospheric pressure and thecover was removed from the casing to provide access to the tube. Thecoated tube was then removed from the casing. The layered structure ofpyrolytically-deposited material was then removed from the interiorsurface of the tube by etching away chemically the tube in aconcentrated solution of sulfuric acid. Each of these layered structurescomprised a first layer of pyrolytic graphite, a layer of pyrolyticboron nitride thereon, and a second layer of pyrolytic graphite thereonand was then machined to the desired size to provide a plurality ofcapacitors.

The edges of the pyrolytic graphite layers were etched away by theoxidizing whereby the boron nitride layer extends over the edges of thepyrolytic graphite layers to prevent arcing therebetween. A pair ofleads were then affixed to the pyrolytic graphite layers or electrodes.Thus, a pyrolytically-deposited capacitor structure was provided by thismethod.

While other modifications of this invention and variations of methodwhich may be employed within the scope of the invention have not beendescribed, the invention is intended to include such that may beembraced within the following claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A method for the preparation of a capacitor device comprising thesteps of:

heating a carbonaceous material at subatmospheric pressure in the rangeof from about 1200 C. to about 2100 C. to convert the carbonaceousmaterial to carbon vapor in the presence of a refractory substrate uponwhich carbon vapor deposits as a layer of pyrolytic graphite, theheating being continued until said layer has reached the desiredthickness,

heating a gaseous medium containing boron atoms and nitrogen atoms atsubatmospheric pressure in the range of from about 1400 C. to about 2000C. in the presence of said layer of pyrolytic graphite to affectpyrolytic decomposition and thereby deposit a layer of pyrolytic boronnitride of desired thickness on said layer of pyrolytic graphite,

heating a carbonaceous material at subatmospheric pressure in the rangeof from about 1200" C. to about 2100 C. to convert the carbonaceousmaterial to carbon vapor in the presence of said overlapping layers ofpyrolytic boron nitride and pyrolytic graphite and thereby deposit alayer of pyrolytic graphite of desired thickness on said layer ofpyrolytic boron nitride,

separating the integrated mass of plural layers from said substrate, andconnecting electrode means to said pyrolytic graphite layers. 2. Themethod substantially as recited in claim 1 including the additional stepof etching away the edges of the pyrolytic graphite layers.

3. The method substantially as recited in claim 1 wherein thecarbonaceous material is methane gas and the gaseous medium containingboron atoms and nitrogen atoms is a mixture of boron trichloride andammonia gases.

4. A method for the preparation of a capacitor device comprising thesteps of:

heating an evacuated enclosure containing a refractory substrate tomaintain the temperature therein in the range of from about 1200 C. toabout 2100 C.,

admitting a carbonaceous gas into said heated enclosure atsub-atmospheric pressure for the conversion of said carbonaceous gas tocarbon vapor and the resultant deposition thereof as a layer ofpyrolytic graphite over said refractory substrate, ceasing admission ofcarbonaceous gas when said pyrolytic graphite layer has reached thedesired thickness,

heating said enclosure to maintain the temperature therein in the rangeof from about 1400 C. to about 2000 C.,

admitting a gaseous medium containing boron atoms and nitrogen atomsinto said heated enclosure at subatmospheric pressure for affectingpyrolytic decomposition thereof and subsequent deposition thereof as alayer of pyrolytic boron nitride over said pyrolytic graphite layer,ceasing admission of said gaseous medium containing boron atoms andnitrogen atoms when said pyrolytic boron nitride layer has reached thedesired thickness,

heating said enclosure to maintain the temperature therein in the rangeof from about 1200 C. to about 2100 C.,

admitting a carbonaceous gas into said heated enclosure atsub-atmospheric pressure for the conversion of said carbonaceous gas tocarbon vapor and the resultant deposition thereof as a layer ofpyrolytic graphite over said pyrolytic boron nitride layer,

ceasing the admission of carbonaceous gas when the last-mentioned layerof pyrolytic graphite has reached the desired thickness,

removing the integrated mass of plural layers formed on said refractorysubstrate,

separating said integrated mass from said refractory substrate, and

connecting electrode means to said pyrolytic graphite layers.

5. The method substantially as recited in claim 4 including theadditional step of etching away the edges of the pyrolytic graphitelayers.

6. The method substantially as recited in claim 4 wherein thecarbonaceous material is methane gas and the gaseous medium containingboron atoms and nitrogen atoms is a mixture of boron trichloride andammonia gases.

References Cited UNITED STATES PATENTS 3,042,565 7/ 1962 Lehovec 29-5893,113,896 12/1963 Mann 2 9-57 9 3,206,331 9/1965 Diefendorf 117-463,221,387 12/1965 Weller et al 29-2542 3,244,953 4/ 1966 Walsh et al29-2542 XR 3,270,254 8/1966 Cohn 29-570 XR 3,317,338 5/1967 Batchelor117-46 3,302,074 1/ 1967 Black 29-570 XR 2,841,508 7/1958 Roup et al317-258 FOREIGN PATENTS 357,510 9/ 1931 Great Britain.

123,619 12/ 1944 Australia.

908,860 10/ 1962 Great Britain.

OTHER REFERENCES New Pyrolytic Materials," by Dr. R. G. Bourdeau inMaterials in Design Engineering, August 1962, pp. 106- 109.

The Condensed Chemical Dictionary, seventh edition, p. 459.

JOHN F. CAMPBELL, Primary Examiner.

R. B. LAZARUS, Assistant Examiner.

US. Cl. X.R. 117-46, 216

